An internal combustion engine (hereinafter referred to as an engine) mounted in a vehicle and the like is equipped with a turbocharger (supercharger) that utilizes exhaust energy. Generally, the turbocharger includes a turbine wheel that is rotated by exhaust gas flowing through an exhaust path of the engine, a compressor impeller to forcibly transfer air in an intake path to a combustion chamber of the engine, and a link shaft to link the turbine wheel with the compressor impeller. With respect to the turbocharger with the structure described above, the turbine wheel disposed in the exhaust path is rotated by the exhaust energy, which allows the compressor impeller disposed in the intake path to rotate in accordance with the rotation of the turbine wheel so as to provide supercharged intake air and forcibly transfer the supercharged air to the combustion chamber of each cylinder of the engine.
As for the turbocharger of this type, there has been known a variable nozzle vane type turbocharger in which supercharged pressure with respect to the exhaust energy is adjustable.
For example, the variable nozzle vane type turbocharger includes a variable nozzle vane mechanism that includes a plurality of nozzle vanes (also referred to as variable vanes) that are disposed in the exhaust gas flow path of a turbine housing and configured to change the flow path area of the exhaust gas flow path, an actuator to provide displacement (rotation) for the nozzle vanes, and the like. The turbocharger changes the opening degree of the nozzle vanes so as to change the flow path area (throat area) between the nozzle vanes adjoined to each other, whereby adjusting the velocity of the exhaust gas introduced to the turbine wheel (for example, see Patent Documents 1 to 3). Thus, the adjustment of the velocity of the exhaust gas is carried out, so that the rotational velocity of the turbine wheel and the compressor impeller can be adjusted, and the pressure of air introduced to the combustion chamber of the engine can be adjusted, which, for example, makes it possible to improve torque responsiveness associated with acceleration and the degree of freedom of conformity in terms of output, fuel consumption (fuel consumption rate), and emission. Also, with respect to the turbocharger, as a method of controlling supercharged pressure, for example, there has been a method in which an exhaust bypass path to bypass the turbine wheel is provided, and a wastegate valve to open and close the exhaust bypass path is provided, and the opening degree of the wastegate valve is adjusted, and the amount of the exhaust gas that bypasses the turbine wheel is adjusted, so as to control the supercharged pressure (see Patent Document 2).